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Volcanic hazards of rift environments Kathy Cashman Volcanic hazards - challenges • To assess hazards quantitatively, we need to know volcanic history in space and time • Vent locations [where?] • Frequency [how often?] • Eruption styles [what are the impacts?] • To mitigate hazards we need to worry about both planning for future activity and response to eruptive crises • To make predictive models, we need to understand complex magmatic systems, that is, the fundamental processes that drive magma storage, ascent and eruption Rift volcanoes in space and time... an overview Fissure vents Cinder cones Shield volcanoes Stratovolcanoes Holocene volcanism Historic eruptions Historic eruptions Fissure vents Fissure eruptions Strombolian violent Strombolian Krafla, Iceland Kilauea, Hawaii • Lava flows • Fed from central volcano • Individual eruptions of short duration • Can’t be distinguished from dyking events Alu-Dalafilla Nov 2008 Image by M Patrick and S Carn Lava flow basics Blocky Pahoehoe Aa Morphology determined by emplacement conditions Surface morphology determined by crystal content and shear rate pahoehoe < ~ 15-20% crystals `a`a > 30-35% crystals Crystal content is a function of original crystallinity and extent of cooling during flow Channelized flow - Two regimes 1. Open channel - steady, mobile central crust - fragmented solid in shear regions HIGH EFFUSION RATES; STEEP SLOPES 2. Insulating tube - continuous solidified roof - efficient delivery of fluid lava LOW EFFUSION RATES; LOW SLOPES Griffiths et al. (2003); Cashman et al. (2006) Regime boundary determined by the relative time scales of advection and cooling (crust formation), as well as the strength of internal convection Historic eruptions Fissure vents Mafic shield Erta Ale Mauna Loa, Hawaii • Predominantly lava flows • Fed from shallow magma storage regions • Individual eruptions can last for months to years to decades Mafic effusive eruptions Intensity (mass eruption rate) 1 km3 Afar 2007, 2009 Laki 1783 105 kg/s Magnitude (erupted mass) Both shield and fissure eruptions can produce flows with a large variation in magnitude; it appears that events of long duration may require large volumes of shallow-stored magma? Historic eruptions Fissure vents Mafic shield Volcanic fields Cinder cone fields Harras of Dhamar, Yemen Paricutin 1943-1952 Michoacan, Mexico Main Ethiopian Rift Strombolian violent Strombolian Cinder cones - effusive Paricutin near Fentale Pioli et al. (2009) Cinder cones - explosive Strombolian Paricutin young cone south of Fentale violent Strombolian Newberry Volcano c. 7000 ybp In many tectonic settings, effusive volume ~ 10 x explosive volume Proportion of tephra increases with MER McKay et al. (in prep) Pioli et al. (2009) Historic eruptions Fissure vents Volcanic fields Mafic shield Silicic centers Fentale Stratovolcanoes Dalafilla • Both explosive and effusive activity • Develop shallow magma storage regions • Erupt a range of compositions Stratovolcanoes - explosive eruptions Mount St. Helens Pyroclastic fall Most common hazards from subplinian to plinian eruptions are deposition of fragmented material from high columns and rapid transport of material via pyroclastic density currents Pyroclastic flow Pyroclastic flows Keleta Fentale Pyroclastic fall s. of Kulumsa Distal hazards Recent eruptions have highlighted the impact of distal ash on aircraft... could also adversely affect shipping Explosive eruptions 1 • Magnitude and intensity are not directly related (duration also important) km3 107 kg/s • Making accurate measurements of volume (and intensity) difficult • Often assessed only for fall deposits data from Carey & Sigurdsson (1989), Carey et al. (2007); Costantini et al. (2009), Houghton et al. (2004), Pistolesi et al. (2011), Pyle (2000); Wong & Larsen (2009) May 1861 eruption of Dubbi Ash fall Felt earthquakes Explosion heard 440 km2 pumice 2.6 km3 mafic lava 2 villages destroyed > 100 fatalities Wiart et al. (2000) Stratovolcanoes - effusive eruptions Lonquimay, Chile Mount St. Helens, USA Fentale Newberry USA Hazards of effusive eruptions Chaiten Obsidian flows may be preceded by violent explosive eruptions Are there other potential sources of volcanic hazard? Historic eruptions Fissure vents Volcanic fields Mafic shield Silicic centers Gedemsa Caldera Crater Lake, OR c. 7700 ybp ~50 km3 DRE Return to the question of the large MT anomaly beneath Dabbahu... what is required to generate a calderaforming eruption? Hydrovolcanic eruptions • Lava flows that enter water • Eruptions through standing water • Phreatic eruptions Maars and tuff cones Hawassa Hydrovolcanism in Afar/Red Sea Zukur Hanish Lake Myvatn, Iceland http://www.phenomenica.com/2009/04/dallol-hell-onearth.html Phreatic eruptions Dallol is an Afari term that means dissolution or disintegration Keir et al. (2009) To Summarize • Rifting environments in general, and Ethiopia specifically, exhibit a wide range of volcanic activity • Although there is no simple spatio-temporal relationship to eruptive activity, we can see that areas of active rifting tend to be dominated by fissures fed by axial volcanoes • More evolved magma where magma supply rates are lower • Question of caldera-forming eruptions • Concern about hydrovolcanic activity 2009 Karbahi II eruption thermal signal SO2 signal from Yirgu et al. preliminary report 2009 Real-time mapping of active flows SAR decorrelation mapping Kilauea 2003-2010 Dietterich et al. (in review) Effusive eruptions - what are the hazards? Hazards mostly related to property... persistent activity may adversely affect air and water quality What determines the hazards? Where? How often? Flow velocity [how fast?] Areal coverage [how far?] Hazard mapping Lava flow hazards maps in Hawaii (or Etna, IT) are based on past activity (%area covered over time)... It is not clear that this is the best approach for an environment like the Afar, where volcanism is distributed in space and may vary widely in composition Where? Once the vent is established, possible flow paths can be predicted using “lava sheds” Kauahikaua et al. (2003) Flow length (km) How far? Average effusion rate (m3/s) Walker (1973) Flow advance rates (how fast?) Time (hours) VER 0.1 km/hr 0.5 km/hr 1 km/hr 10 km/hr Distance (km) Kauahikaua et al. (2003) How to measure MER? Making direct measurements on active flows is challenging... Thermal Imaging (time-averaged discharge rate) Vicari et al. (2011) Depth (m) Using observed flow features? Velocity (m/s) Fr = 1 V = √gd Flow branching Flow branching also affects flow advance rate V = 0.07 km/hr V = 0.15 km/hr V = 0.08 km/hr Episode 5 data from Wolff et al. (1988) Topographic confinement can increase flow length Koae fault zone Manda Hararo Summary - Lava flow hazards Pele dancing by Katia Krafft Hazards relate primarily to property damage... associated hazards include fires, volcanic gas, and ‘rootless’ explosions Hazard impacts depend on magnitude and intensity (control area covered and rates of flow advance) Remote sensing techniques for monitoring are improving rapidly 57